preamp, v2

NB: The use of the software on this page is deprecated
in favour of more recent development lines.

This page presents a LADSPA plugin approximating the characteristics
of soft and hard harmonic distortion of a 12AX7-based valve preamplifier stage,
as well as an emulation of typical valve amplifier
linear filtering characteristics. The plugin has been reworked from
a (somewhat crude) previous version.

The initial hi-pass filter (an equivalent is found in actual circuitry)
prevents the lower frequency content from dominating the tone evolution
of the model (no 'auto-wah' effect) at high gain settings.

The first transfer function (polynomial, shaped in the likeness of data
obtained from an emulated valve preamp circuit) causes gentle
harmonic distortion and compression.

From data sampled off a real amplifier it can be seen that the operating
point (0 equivalent) of the circuitry is in constant movement. This
behaviour is approximated (albeit a bit roughly) by a low-frequency
(0.4 Hz sine) LFO.

To minimize aliasing artefacts in the unit's output, the clipping transfer
function is applied to an oversampled version of the input signal.
The filters for both the interpolator and the decimator
stages have been implemented as precomputed IIR filters which give the unit's
frequency response a subtle mid-range frequency boost which is
characteristic of the modeled circuit, and assumed to be characteristic
of guitar amplifiers in general.

The transfer polynomial
gives smooth and continuous clipping characteristics. Up to very high gain
settings, the asymmetry of
the transfer function provides a colourful mix of partials strongly
resembling that which makes 'valves sound better'. It also adds DC
content, which is removed by the final DC-blocking filter.

Changes

Download

Your feedback is welcome.
Please let me know
if/how the plugin works for you, and what you would like to see
improved.

Installation

$ tar xvfz preamp.tar.gz
$ cd preamp
$ make
# make install

How to Use

There are two control ports:

gain - behaves much like the gain knob on a traditional
preamplifier. gain settings below 1 are in linear scale,
above 1 exponential. The hard-clipping point is reached at around 0.6+
for a normalized input signal, but it kicks in very gently.

fc - the -3 dB frequency for the initial hi-pass filter.
High fc settings cause the output to sound 'thinner'.
The default value is 440 Hz.

The unit applies final attenuation/gain to the signal to smoothen the
average loudness of the output over the gain range.

Sample Rate Dependencies

The oversampling filters employed in the unit are designed for
specific sample rates.

FS (kHz)

oversample

44 - 48

4x

88 - 96

2x

176 - 192

none

At different sample rates, the
frequency response of the plugin is shifted, and aliasing attenuation
can be reduced.

You may want to route the unit's output through an EQ and a
cabinet emulation
like 'matched' to recreate more of the
characteristics of the real thing.

Future versions of this plugin may or may not produce a different sound.